This February 2012 provided by researcher Mikkel Winther Pedersen shows a southward view down Cline River in Alberta, Canada, where retreating ice sheets created an ice-free corridor more than 13,000 years ago.(Mikkel Winther Pedersen via AP/Wiki Commons)

Researchers have found evidence that the first Americans migrated south from Alaska via the Pacific coast, rather than a route hundreds of miles inland along the Rocky Mountains.

The colonization of the Americas began after people arrived from Siberia. They crossed an ancient land bridge called Beringia into Alaska. Huge ice sheets largely blocked the way south. But a gap in western Canada was long thought to provide an ice-free corridor for migration into the continent.

That idea ran into a problem, as archaeologists documented human presence in the Americas at earlier and earlier times. The corridor appeared some 15,000 to 14,000 years ago as the ice sheets retreated. Now studies suggest that people had reached South America by at least 14,700 years ago. Even if one accepts the earliest date for the corridor, it's hard to believe the migration could have gone so far south, so fast.

So in recent years, many scientists have concluded that the first southward migrants traveled along the Pacific coast instead. They came either in boats or on land.

The new research was released by the journal Nature. It casts further doubt on the inland corridor. It suggests that even after the corridor appeared, it wasn't suitable for migration until about 12,600 years ago.

That's because it lacked plants and game. People would need these to sustain themselves on the long journey, researchers concluded.

The paper was produced by Eske Willerslev of Cambridge University and the University of Copenhagen in Denmark and coauthors. The paper analyzed pollen and traces of animal DNA from ancient sediments of two lakes near Fort St. John in northeastern British Columbia. That general area is where the corridor last opened.

The paper follows another recent study of the corridor that also concluded it became habitable too late for the first migration south.

The earlier paper used a different method to assess habitability, and one of its authors, Beth Shapiro of the University of California, Santa Cruz, said in an email that "it's great when two different (approaches) agree on an issue that has been unresolved for such a long time."

In response to "America's first settlers came via Alaska," I agree that the first settlers to come into North America came from Alaska. One reason I agree is because Beringia (also called the land bridge) is the only way that the people could have crossed, if they didnt go by boat. Another factor is that people didn't invent boats yet so they couldn't have come from the area around Greenland. It says in the article that there was a narrow corridor that ran along the Pacific coast that the people could have traveled across. The only problem was that the corridor lacked of plants and game (wild animals). A third reason is that the people entered Alaska, headed south, and reached South America in 300 years. That would have taken a lot longer if you had to cross the country. Even though the theory before was that they came from around Greenland by boat, I think that they crossed the land bridge and headed south from Alaska.